Alternanthera Philoxeroides
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''Alternanthera philoxeroides'', commonly referred to as alligator weed, is a native species to the temperate regions of
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, which includes
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,
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and
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. Argentina alone hosts around 27 species that fall within the range of the genus ''
Alternanthera ''Alternanthera'' is a genus of flowering plants in the family Amaranthaceae. It is a widespread genus with most species occurring in the tropics, tropical Americas,Sánchez-Del Pino, I., et al. (2012)Molecular phylogenetics of ''Alternanthera'' ...
''. Its geographic range once covered only the Parana River region of South America, but it has since expanded, having been introduced to over 30 countries, such as the
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,
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,
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and many more. This
invasive species An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...
is believed to have been accidentally introduced to these non-native regions through sediments trapped by, or attached to, tanks and cargo of ships travelling from South America to these various areas.


Description

''Alternanthera philoxeroides'' can thrive in both dry and aquatic environments and is characterized by whitish, papery flowers along its short stalks, irregular, or sprawling hollow stems, and simple and opposite leaf pattern sprouting from its nodes. It is also considered a
herbaceous plant Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition o ...
due to its short-lived shoot system. It produces horizontal stems, otherwise known as
stolon In biology, a stolon ( from Latin ''wikt:stolo, stolō'', genitive ''stolōnis'' – "branch"), also known as a runner, is a horizontal connection between parts of an organism. It may be part of the organism, or of its skeleton. Typically, animal ...
s, that can sprout up to in length and thanks to its hollow stems, floats easily. This results in large clusters of stem amassing and create dense mats along the surface. Bisexual flowers are small, white, and born on dense, axillary spikes (type of
raceme A raceme () or racemoid is an unbranched, indeterminate growth, indeterminate type of inflorescence bearing flowers having short floral stalks along the shoots that bear the flowers. The oldest flowers grow close to the base and new flowers are ...
). Presence of a peduncle is a key trait that distinguishes this species from the sessile
inflorescences In botany, an inflorescence is a group or cluster of flowers arranged on a plant's stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a main axis ( pe ...
of ''
Alternanthera sessilis ''Alternanthera sessilis'' is a flowering plant known by several common names, including sissoo spinach, Brazilian spinach, sessile joyweed, dwarf copperleaf. It is cultivated as a vegetable worldwide. Distribution The plant occurs throughout th ...
''. While seed production has been observed in its native range, there is currently no record of viable ''A. philoxeroides'' seeds in the introduced range. The weed's intricate root system can either allow them to hang free in the water to absorb nutrients or directly penetrate the soil/sediment and pull their nutrients from below.


As an invasive species

''Alternanthera philoxeroides'' is considered a major threat to ecosystems because of the adverse effects it poses on both aquatic and terrestrial environments, as well as the negative influence it has on society. The species features on the list of invasive alien species of Union Concern since 2017. This means that import and trade of this species is forbidden in the whole of the European Union.


Impacts on vegetation

The presence of this invasive species disrupts the natural flow of water due to the dense mats created by its clusters of stems. It out competes the native vegetation for space and solar energy through these dense mats because they form large clusters and limit the amount of light that submerged vegetation receives. These compact clusters of stems also disturb the regular exchange of gases that occurs underneath the surface that directly influences aerobic processes, such as
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. Aside from driving down the population of native aquatic vegetation, ''A. philoxeroides'' can also influence the growth and yield of crops in pastures and fields. These dense mats can affect the natural flow of water that is used in irrigation systems and as well as affect the quality of the water by increasing the sedimentation present in the water. Both of these factors must remain undisturbed for crops to grow well, and thus to provide a healthy yield for farmers, which will be discussed in the later section on its impacts on society. On top of this, the likelihood of flooding is higher due to the impaired drainage caused by the dense mats, which in turn can also damage crops. This invasive species also has other negative impacts on the environment.


Impacts on animals

As mentioned, the compact mats formed by this species can drive down the population of native vegetation in the environments it invades. This then becomes a major issue for native herbivores because their food source declines. In addition, the dense mats present a challenge for the native wildlife by acting as a barrier between them and natural water sources. However, even if they can reach the water, they are still at risk because the water quality can be contaminated by the increased sediments. Thus, just as it did with the native vegetation, ''A. philoxeroides'' is also driving down the populations of the native wildlife as well.


Impacts on society

Dense mats formed by this species influence the natural flow of water, which can impede various recreational activities, such as boating and fishing. The disruption of flow can also have a negative impact on infrastructure when it comes to energy, such as the use of hydro-electric dams to power generators. The dense mats also present suitable ecological conditions that mosquitoes can thrive off of. This is a commensal relationship between mosquitoes and ''A. philoxeroides'' because mosquitoes receive a breeding ground and the plant gains nothing nor loses anything. The increased population of mosquitoes can bring an increased risk of
mosquito-borne disease Mosquito-borne diseases or mosquito-borne illnesses are diseases caused by bacteria, viruses or parasites transmitted by mosquitoes. Nearly 700 million people contract mosquito-borne illnesses each year, resulting in more than a million death ...
s in humans. Furthermore, the dense mats produced by ''A. philoxeroides'' may not present suitable ecological conditions for native species or humans to thrive off.


Reproduction and dispersal

This invasive plant depends solely on vegetative means to reproduce and disperse itself in the area it has invaded and established its roots. In its native geographic range, the species spreads to through means of producing viable seeds; however, it has been observed within its non-native ranges that it rarely produces viable seeds. To accommodate this, the ''A. philoxeroides'' reproduces through fragmentation; the plant can regenerate itself from small portions of stems or small leaf cuttings. These small fragments of the plant can then be dispersed through human means and natural means; once it is dispersed, the fragments can then find suitable ecological conditions and root themselves and regenerate.


Human dispersal

In effort to eradicate the species by manual means, such as mulching or pulling them out, if not removed efficiently, small stem fragments can be displaced to new areas. Soil movement caused by earthmoving machinery is another example of how humans influence the dispersal of the plant.


Natural dispersal

In aquatic environments, the ''A. philoxeroides'' can easily disperse its fragments by being sucked into the path of the waterways. In terrestrial environments, the small fragments of stems and leaf cuttings can be dispersed through the natural movement of soil caused by erosion. Thus, the geographic range of this invasive species can easily be expanded by any means necessary due to its ability to regenerate from practically nothing.


Methods of control


Preventive measures

Early detection is the best bet to ensure that the invasive species does not successfully colonize a non-native region because of its ability to regenerate and propagate from small portions of its stem or leaf cuttings. However, when that is not possible, the best that can be done is to limit and control the presence of ''A. philoxeroides'' in an area. ''Alternanthera philoxeroides'' can only establish itself in shallow waters no deeper than , so one method of control is to erect barriers in shallower areas to limit the amount of suitable space the plant has. When it comes to terrestrial environments, overpopulating the area with native species can limit the suitable space available for it. However, this method is only effective before the invasive plant has asserted itself in an area. Lastly, as previously mentioned, this plant is only able to produce viable seeds in its native geographic range and not in the areas it has invaded. Learning why that is - what ecological conditions make it produce only sterile seeds - could be key to developing further preventative measures against it.


Biological measures

Insects have been released for the biological control of ''A. philoxeroides''. The most successful and widely used is ''
Agasicles hygrophila ''Agasicles hygrophila'' is a species of leaf beetle known by the common name alligator weed flea beetle. It has been used successfully as an agent of biological pest control against the noxious aquatic plant known as alligator weed (''Altern ...
'' commonly called the alligator weed flea beetle; it has been released for biocontrol in
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,
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,
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,
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, and the
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. However, their effectiveness is limited due to their inability to survive through temperatures lower than . ''
Amynothrips andersoni ''Amynothrips andersoni'' is a species of thrips known as alligator weed thrips. It has been used as an agent of biological pest control against the noxious aquatic plant known as alligator weed (''Alternanthera philoxeroides''). This thrips ...
'', the alligator weed thrips, and ''Vogtia malloi'', the alligator weed stem borer, have also been released in the United States. These species result in immediate wilting and limit ''A. philoxeroides'' reproduction by colonizing its stems. A variety of chemicals have been shown to be effective in controlling the plant, the most useful of which include
glyphosate Glyphosate (IUPAC name: ''N''-(phosphonomethyl)glycine) is a broad-spectrum systemic herbicide and crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by EPSP inhibitor, inhibiting the plant enzyme 5-en ...
,
triclopyr Triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid) is an organic compound in the pyridine group that is used as a systemic foliar herbicide and fungicide. History Triclopyr triethylammonium (TEA) was first registered in 1979 in the United ...
,
fluridone Fluridone is an organic compound that is used as aquatic herbicide often used to control invasive plants. It is used in the United States to control hydrilla and Eurasian watermilfoil among other species. Fluridone is sold as a solution and as ...
, imazamox, and
imazapyr Imazapyr is a non-selective herbicide used for the control of a broad range of weeds including terrestrial annual and perennial grasses and broadleaved herbs, woody species, and riparian and emergent aquatic species. It is used to control annu ...
; however, they must constantly be applied to be successful.


Legality of sale and shipment in the United States

In 1956, ''A. philoxeroides'' was banned for sale or shipment in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, subject to a fine and/or imprisonment. This law was repealed by H.R.498
16th Congress (2019-2020) 16th Congress may refer to: * 16th Congress of the All-Union Communist Party (Bolsheviks) (1930) * 16th Congress of the Philippines (2013–2016) * 16th National Congress of the Chinese Communist Party (2002) * 16th National Congress of the Kuomin ...
on January 23, 2019.


See also

*''
Ludwigia grandiflora ''Ludwigia grandiflora'', the water primrose, is an aquatic plant of the order Myrtales. It is closely related and easily confused with '' Ludwigia hexapetala''. The two species can be distinguished at a chromosomal level, because ''L.grandiflor ...
''


References


External links


The evolutionary history of an invasive species: alligator weed, ''Alternanthera philoxeroides''
XII International Symposium on Biological Control of Weeds
Pest Plant Risk Assessment: Alligatorweed
Biosecurity Queensland, Department of Employment, Economic Development and Innovation
Species Profile - Alligatorweed (''Alternanthera philoxeroides'')
National Invasive Species Information Center,
United States National Agricultural Library The United States National Agricultural Library (NAL) is one of the world's largest agricultural research libraries, and serves as a national library of the United States and as the library of the United States Department of Agriculture. Locate ...
. Lists general information and resources for Alligatorweed.
Alternanthera philoxeroides (Mart.) Griseb.
Medicinal Plant Images Database (School of Chinese Medicine, Hong Kong Baptist University) {{Taxonbar, from=Q1472735 philoxeroides Aquatic plants Flora of Southern America